The present invention relates to charge disposition print heads of the type wherein selectively controlled electrodes, generally arranged at two or more levels in a laminated construction, are disposed to define a matrix array of charge-generating or gating points from which charge carriers are directed at an imaging or latent imaging member moving along a scan direction past the head. Such print heads allow a thin rectangular strip of charge generators to deposit an image of arbitrary length, with high resolution, on an imaging member as it moves past the head, and they are readily adapted to print computerized graphic image and text data.
Print heads of this type are described in U.S. Pat. Nos. 4,160,257; 4,992,807; 5,278,588; 5,159,358; and many others. In the print heads described more particularly in the aforesaid patents, long driver electrodes spanning a page format on an insulting sheet are activated with an RF signal of up to several thousand volts amplitude while lesser bias or control voltages are applied to other electrodes on the other side of the sheet to create localized charge source regions located at or near crossing points with the driver electrodes and allow charge carriers to escape from the glow or discharge regions and be accelerated to an imaging member. These print heads may be configured to deposit either positive or negative charge, and the negative charge may consist partly or entirely of either ions or electrons. These heads are configured so that the charge deposited by each discharge locus forms a small dot-like latent charge image on the imaging member as it moves past. Each raster scan of the head electrodes thus fills a narrow image strip, with the totality of image strips forming an image page.
In printing devices using this type of head, the RF-driven discharge generation line may extend generally along the length of the print head, spanning many of the control electrodes which cross them at an angle. In one commercial embodiment, by way of example, 20 parallel RF lines extend the width of a print page, and these are crossed by 128 oblique control electrodes, known as finger electrodes. During the time when one RF line is activated by a burst of approximately 5 to 25 cycles of a one-half to ten MHz drive signal with a peak-to-peak amplitude of several thousand volts, those finger electrodes which cross the RF line at the desired locations are selectively biased to project charge dots from the head at the pixels where a print image is to be formed. Each finger is effective to project up to twenty charge dots, arranged along its length, corresponding to the twenty adjacent RF drive lines crossing the finger. The last projected dot (e.g., from drive line 20) of one finger may be adjacent to the first dot (the crossing point with drive line 1) of the neighboring drive. The drive electrodes generally extend at a pitch which allows the electrode-crossing points to be relatively widely spaced along the length of the finger, yet have a much smaller spacing in the cross-scan direction of imaging. Thus, by actuating different drive lines having electrode crossing sites on the same finger but at slightly different times as the imaging member is moved in a scan direction past the print head, a finger may deposit dots closely adjacent to each other on a single print line, or to dots of successive lines.
Other forms of charge deposition print head are also known. These may include structures wherein electrodes spaced on opposite sides of an insulating plate or sheet surround apertures through the sheet which define the sites at which charge carriers are gated through to land on an imaging member. Such constructions may be used to define image points and to control the flow of ions from a plasma chamber, or may even be used to directly gate charged toner particles onto an imaging member in a direct printing process. In addition various charge dot generating structures have been proposed utilizing semiconductor solid state electron emitter arrays, as well as electron field emitter arrays which rely primarily on microdimensioning to achieve effective field strengths for emitting electrons.
Returning to the first type of print head discussed above, these are generally operated at a relatively small gap of about one-quarter millimeter from the image receiving sheet, belt or drum, and they are biased with respect to the imaging member to maintain a relatively high electrostatic acceleration field which transports the charged particles across this gap. The amount of charge or charged particles which must be deposited to form an effective imaging dot is generally so great as to result in a considerable build-up of charge at the dot locus on the charge-receiving surface of the imaging member, relative to the magnitude of the acceleration potential. Thus, as a latent dot charge is formed, a local electric field develops which tends to deflect later arriving charge carriers directed at or near that dot. This effect results in "blooming" or enlargement of individual dots, such as described in the aforesaid U.S. Pat. No. 5,278,588, and various approaches are taught therein for addressing the precision of dot placement and image control to overcome deleterious the effect of dot blooming on image resolution. This surface charging effect also slightly deflects dots which are aimed nearby. This related effect, known as "Venetian blinding". occurs when electrodes are actuated to lay down a latent charge dot on the imaging member at a position closely adjacent to one or more charge dots which have already been laid down along a line or region. In this case, the already deposited charge deflects the incoming charge carriers so that the subsequent dot is shifted laterally and positioned wrong. Since the RF lines are few in number and are actuated in a generally fixed sequence, an effect similar to aliasing also occurs, wherein the print head generates a ripple or line of misplaced dots that appear as a streak or an anomalously light or dark band periodically crossing the face of the print. One approach to correcting Venetian blinding is taught in commonly owned U.S. Pat. No. 5,450,103. That approach involves providing a second array of electrodes which can be actuated to correct the small deflection fields existing at the surface of the imaging member, so that charge trajectories remain on target and deposit dots of uniform size.
Finally, a third effect arises due to variations in spacing of the print head from the image-receiving member. If the image receiving member is a curved drum, or a belt which passes over a drum opposite to the print head, then the RF line or lines closest to "top dead center" of the drum will be closest to the imaging member, while RF lines adjacent thereto and outward from the center will be further away from the imaging surface due to curving-away of the drum surface. The increased gap results in lower extraction or acceleration field strengths, with the result that less charge is emitted by and deposited opposite to these outer electrodes. The periodic actuation of RF drive lines and scanning of the drum past the head therefore results in a pattern of weak and strong charge dots which can also give macroscopically-visible banding or texture to the developed charge image.
Thus, despite the apparently high degree of symmetry of existing charge deposition print heads, a number of macroscopically visible irregularities are produced in the images which they deposit.
These problems were partially solved by introducing an interleaved finger electrode configuration as described in U.S. Pat. Nos. 4,999,653 and No. 5,006,869; however using this arrangement some anomalously lighter or darker streaks persisted.
It would be desirable to achieve uniform dot deposition without requiring coordination of additional electrodes and without losing the basic simplicity of the raster electrode actuation employed to drive charge deposition heads of the prior art.